The present invention relates to a process for separating para-cresol and meta-cresol from a mixture of methylated and ethylated phenols. More particularly, the process is directed to obtaining separately high purity para-cresol and meta-cresol isomers from a mixture of methylated and ethylated phenols.
Mixtures of methylated and ethylated phenolic compounds may be derived from coal as products of coal carbonization, as the middle oil from the hydrogenation of coal, from the liquefaction of coal, or from petroleum as alkaline extracts of cracked petroleum distillate. Some of the methylated and ethylated phenols in a mixture derived from these sources have similar physical properties. These similarities in properties make it difficult to separate some of the phenolic compounds from each other. Para-cresol and meta-cresol are compounds that are usually present in mixtures of methylated and ethylated phenols, and they are difficult to separate from the other methylated and ethylated phenols and from each other. For example, para-cresol is difficult to separate from meta-cresol since both compounds have similar boiling points, namely, 201.8.degree. C. and 202.8.degree. C., respectively. The similarity of boiling points of these two compounds precludes their separation by simple distillation and also makes fractional distillation a complicated procedure.
There are several known methods for separating para-cresol and meta-cresol isomers from a mixture containing the isomers, such as a commercial mixture having 40-65 percent of meta-cresol. These methods include treatment of a mixture containing meta- and para-cresol isomers with complexing agents such as urea, sodium acetate, oxalic acid or the like and separation of the meta- or para-cresol isomers in adduct form. Other methods include the formation of a solid complex between a reagent and one cresol isomer. The reagents used include ortho-toluidine, oxalic acid and hexamethylenetetramine. Still other methods for separating meta- and para-cresol isomers involve azeotropic distillation with benzyl alcohol, or selective solvent extraction with methanol-ligroin, or hydrolysis of the sulfonic acid of meta- or para-cresol, or dibutylation followed by distillation and debutylation. Another recently suggested method for separating meta-cresol and para-cresol isomers takes advantage of the different melting points of the compounds and involves subjecting the meta- and para-cresol isomer mixture to crystallization at atmospheric pressure of not less than about 300 atmospheres. Only a few of the foregoing methods of separation have any commercial potentialities, the process usually employed being the butylation method. A more efficient method is thus desired to obtain high purity para-cresol and high purity meta-cresol from mixtures of methylated and ethylated phenols containing compounds such as ortho-ethyl phenol, xylenol and other methylated and ethylated phenols having similar boiling points to para-cresol and meta-cresol.
An article entitled "Rapid Separation of Organic Mixtures by Formation of Metal Complexes", Journal Organic Chemistry, Volume 40, No. 9, 1975, Sharpless, Chong and Scott, describes a convenient and efficient technique for resolving alcohol mixtures. The technique involves preferential complexing of an alcohol by calcium chloride or manganese chloride, examples of useful alcohols being cis- and trans-4-tert-butylcyclohexanol, geraniol, and cyclohexanol. The article further notes that other alcohols, such as large and hindered alcohols, form complexes very slowly, and that the speed of complex formation can be increased by using a small amount of a lower aliphatic alcohol as a catalyst for complexing.